The present invention is directed to communication systems specifically cellular communication systems.
Wireless communications systems have increased in complexity and sophistication. A typical wireless communication system is divided into discrete areas which may or may not overlap known as cells. Each cell is serviced by a base station located within the cell. The base stations are coordinated by a mobile telecommunication switching office (MTSO) or mobile switching center (MSC). The MTSO or MSC provide a communication link to a Public Switching Telephone Network (PSTN). In addition, individual base stations may be controlled by a base station controller which controls the communication between the base station and the MTSO or MSC.
A cellular device typically communicates with the base station and enables an end user to utilize the network. Each base station communicates with the cellular device within communication range of the base station on a group of frequencies or channels. These channels can be reused in distant cells. The distant cells are called co-channel cells. The distance between co-channel cells must be far enough to reduce interference in the co-channel. In addition, base stations located in adjacent cells typically communicate on different frequencies to avoid interference problems. The frequency coordination and allocation between the base stations is usually performed by the MTSO or MSC.
As a cellular device moves from a first cell to a second cell, a first base station in the first cell will stop servicing the cellular device and a second base station in the second cell will begin to service the cellular device. The change of service from the first base station to the second base station is accomplished through a technique known as handoff. In other words, the first base station will typically hand off the communication and control of the cellular device to the second base station. Specifically, handoff is accomplished when a cellular device traveling from a first cell to a second cell is directed to change frequencies from the transmitting frequency of the base station in the first cell, to the transmitting frequency of the base station in the second cell. Typically, the base station in the first cell will instruct the cellular device to retune to the transmitting frequency of the base station in the second cell after receiving cell assignment instructions from the MTSO or MSC.
The cellular device retunes to a new channel that carries the information required to communicate with the network. A specific channel will have both voice information and control information for control signaling between the base station and the cellular device. In addition to the channels that are in use, there may also be reserved channels for handling handoff traffic. These reserved channels may be statically assigned to a specific cell or dynamically reallocated amongst cells. When there are no additional channels to assign to a cellular device, the call from the cellular device is blocked or dropped.
In a typical wireless communication system, traffic varies with time and location. At a specific time traffic may be heavy at a first location while traffic at a second location may be light. At a second time, traffic at the second location may be heavy while traffic at the first location may be light. As a result, planning and allocating channels in this dynamic wireless environment is challenging, since frequency spectrum and channels are a precious resource in cellular systems.
Several channel allocation schemes have been developed to address channel allocation amongst cells in a cellular system. Most of these cellular schemes are not suited for addressing the needs of the dynamic traffic patterns found in a typical cell. For example, in a Fixed Channel Allocation Scheme (FCA) channels are assigned to specific cells and there is no sharing of channels between cells. In FCA, the communication channels do not dynamically change to accommodate the varying demand across cells. The overall channel resources in the network are not optimized and there is low channel efficiency. As a result, in FCA based schemes some channels may have a high occurrence of blocked calls even when there are channels available for use in other cells.
Several techniques have been developed to address some of the problems associated with FCA schemes. Among these schemes are Dynamic Channel allocation (DCA) schemes and Adaptive Channel Allocation (ADA) schemes. In a DCA scheme a centralized base station has a group of channels for use. These channels can be allocated in real-time, on a call by call basis, across adjacent cells, as demand requires. If the demand in one cell increases beyond the other cell, DCA schemes allow the base stations to allocate channels to accommodate the additional demand. However, every event in a system using DCA may change the channel status in the system therefore, each event requires re-computing and reallocation of a channel. For example, a new call arrival, handoff or ending call requires that the MTSO or MSC re-compute and reassign channels. This results in a large degree of complexity for the base station and the network switch, which are required to re-allocate the channels. Therefore, an enormous amount of computing power is required to make a DCA system realizable.
In an Adaptive Channel Allocation (ACA) scheme, frequency assignment is done in a distributed manner. Each cell scans the other cells to determine where channel bandwidth is available. The base stations in the cells then compete for the channel bandwidth. However, with multiple cells all competing for the same channels, there are often conflicts in the ACA based systems. For example, more than one base station may request the same channel. When two base stations each try to acquire the same channel one of the base stations will find that the channel is already acquired or blocked. The base station will then have to search for another channel. Assessing that a channel is used and then acquiring an alternate channel adds complexity and timing delays to ACA based systems.
As mentioned above, the base stations using an ACA scheme acquire unused channels by searching across the allocated spectrum for free channels. The Radio Frequency (RF) tuning process used to accomplish this task is often slow and simulation has shown that as a result of this slow scanning process, the ACA schemes are not able to keep up with the dynamic changes in traffic flow.
It would be beneficial to take advantage of the advantages of both the DCA and ACA schemes while avoiding their disadvantages. For example, it would be advantageous to avoid the computing complexity of the DCA scheme but have the centralized channel allocation that the DCA scheme offers. In addition, it would be advantageous to avoid the channel requesting conflicts found in the ACA scheme, but still scan channels for the optimum channel assignment.
The present invention is directed to a method and apparatus for allocating channels in a wireless communication network. In the present invention the deficiencies of the DCA and ACA schemes are minimized while taking advantage of the strengths of each technique. A channel allocation scheme is disclosed that utilizes centralized channel allocation but avoids computing complexity and channel conflicts while scanning channels for the optimum channel assignment. A method of allocating channels is disclosed in which cell operational measurement data such as traffic density information and mobility user information, is collected from wireless terminals in a first cell. The base station in the first cell then scans the channels in other cells to determine if any free channels are available. The free channels are then ranked in a priority list. A calculation is made by a switch connected to the base station to determine the amount of channels that are required. Using the traffic information and the priority list, channels are then acquired for use by the base station in the first cell.
Cells that have excess channels are calculated during a time period T. The excess channels are then released by the base station that is holding the channels. The released channels are then allocated to new base stations for use in different cells. The cells that have excess channel capacity are known as channel-spare cells and the cells that have limited channel capacity are known as channel-scarce cells. Channels in the method and apparatus of the present invention are determined over varying time periods. Therefore assigned channels, excess channels and requested channels are all determined over a time period that changes in response to changing traffic patterns. Once the assigned channels, the excess channels and the requested channels are determined, channels are allocated in response to the assigned channels, the excess channels and the requested channels.